restore: reject invalid combinations with --staged

git-checkout rejects plenty of invalid option combinations. Since
git-checkout is equivalent of either

    git restore --source --staged --worktree

or

    git restore --worktree

that still leaves the new mode 'git restore --index' unprotected. Reject
some more invalid option combinations.

The other new mode 'restore --source --worktree' does not need anything
else.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -433,6 +433,16 @@ static int checkout_paths(const struct checkout_opts *opts,
 		die(_("'%s' must be used when '%s' is not specified"),
 		    "--worktree", "--source");
 
+	if (opts->checkout_index && !opts->checkout_worktree &&
+	    opts->writeout_stage)
+		die(_("'%s' or '%s' cannot be used with %s"),
+		    "--ours", "--theirs", "--staged");
+
+	if (opts->checkout_index && !opts->checkout_worktree &&
+	    opts->merge)
+		die(_("'%s' or '%s' cannot be used with %s"),
+		    "--merge", "--conflict", "--staged");
+
 	if (opts->patch_mode) {
 		const char *patch_mode;